In prior art systems for plotting and scanning graphic images, a graphic image is plotted or scanned through the use of a beam of light that is caused to travel over a target, such as film or a document, in a precise, predetermined path. At each point along this path, the light beam, which is typically a laser, strikes a particular region on the target corresponding to an image pixel. In a plotting operation, the striking of this region of the film target causes it to become exposed. In a scanning operation, the striking of this region of the image target causes a certain amount of the light beam to be reflected. The reflected light is measured, and is used to generate image data that is stored and processed, as required.
In typical prior art systems, mechanical and electronic means are provided to move the light beam along the path while the target remains stationary. In other prior art systems, in addition to manipulating the light beam, the target is also moved to facilitate travel of the light beam along the predetermined path.
For example, in so-called "flatbed" systems, the target is mounted to a flat target surface, and the light beam is maneuvered over the target using movable mirrors or other means known in the art.
In an internal drum system, the target is mounted to the inside of a hollow cylindrical drum. Travel of the light beam along the target path is typically accomplished by moving a light beam assembly along the inside length of the drum, i.e., the "x axis," and rotating the light beam assembly, i.e., advancing the beam along the image's "y-axis."
Finally, in an external drum system, the target is mounted to the outside surface of a cylindrical drum. Similar to the internal drum system, means are provided to advance the light beam along both the image's x-axis and y-axis. In an external drum system, there is a choice as to how to integrate movement along the x- and y-axes. In one approach, the drum is allowed to complete an entire rotation before the light beam is advanced one unit along the length of the drum. This has the advantage of preserving orthogonality, but slows down processing because of alignment problems, and because a certain amount of "debouncing" time is required after the light beam has been advanced the one unit. In a second approach, both the drum and the light beam are advanced continuously. This results in a helical scan path around the cylinder, which has the advantage of speed, but creates a distortion in orthogonality.
All three prior art scanning/plotting systems suffer from a common drawback. The precision required in advancing the light beam along its predetermined travel path, and the spot size of the beam required for high resolution, impose speed limitations on scanning/plotting systems. Thus, even though the scanning or plotting data may be available and can be processed at extremely high rates, these limitations of a scanning/plotting system determine how fast a particular document can be processed.